The government's case against Osama bin Laden for the September 11 atrocity does not prove, in Iain Duncan Smith's inapt words, that he is "guilty as charged". It demonstrates that there is sufficient evidence to put him on trial, a fact which requires the creation of a court capable of trying him fairly. Otherwise the justice that the US and its allies seek will be that of the lynch mob, not of international law.

The prime minister's initiative in tabling this indictment of Bin Laden and the Taliban is welcome. It is not "evidence", so much as a convincing argument based on similar facts, matters of record, intelligence analysis and reasonable suspicions. It shows in bloodcurdling detail that Bin Laden has consistently incited the murder of Americans and is assuredly implicated in the 1998 US embassy bombings.

There is much that points to his organisation as responsible for September 11 and to dismiss it all as circumstantial misses the point: in proving conspiracy, circumstantial evidence is often more credible than fallible human testimony. But to amount to proof beyond reasonable doubt, it must go further than the document's assertion that no other terrorist network is known to have the capacity to have carried out the attacks. What the government has presented to parliament is a prima facie or presumptive case, sufficient for extradition and for putting the man on trial. But at which court?

At this point an embarrassed silence descends. The "justice" that Mr Blair correctly demands cannot sensibly or dispassionately be delivered by a New York jury. Any jury verdict of "guilty" without reasons will not convince, and the spectacle which would follow in America - a death sentence by lethal injection or in the electric chair - is too grotesque to contemplate. If Bin Laden is to be fairly tried, it must be by an international court, with distinguished jurists (including Muslim judges) giving a logically reasoned decision.

In The Hague, just such a court exists for perpetrators of "crimes against humanity" in former Yugoslavia. It has developed reasonably fair procedures for evaluating the kinds of evidence upon which the government's charge heavily relies, namely electronic intercepts and other fruits of secret intelligence gathering. The UN security council in its present mood would readily accept any US request to set up a similar tribunal for those accused of masterminding what was in truth a crime against humanity.

The urgency of putting in place some such "Lockerbie solution" becomes plain from reading between the lines of yesterday's government statement, which seeks to lay the foundation for an attack on Afghanistan. It is most convincing when it outlines the extent to which the Taliban regime has collaborated with Bin Laden's terrorist training enterprises. In international law, every state has a duty to prevent its territory being used for unlawful attacks on other states. In June of this year the US formally reminded the Taliban of that duty and warned that it would be held responsible for Bin Laden. Its refusal to surrender him now that a prima facie case has been made entitles the US to resort to force, although initially only for the limited purpose of apprehending Bin Laden and destroying his camps. Only if the Taliban counterattacks against a legitimate operation of such a kind is the US justified in waging a wider war on Afghanistan.

What yesterday's statement demonstrates beyond any doubt is not that Bin Laden ordered Black Tuesday but that ever since 1996 the state of Afghanistan has knowingly and truculently connived at his unlawful plotting against other states, a position its government still maintains by refusing to take any action against him. This provides a mandate for the US and its allies to breach Afghanistan's sovereignty. But the right of self-defence is not a right to retaliate or seek reprisals. It is limited by rules that the force used must in no way target civilians and must be proportionate to the legitimate object of the mission.

But what, exactly, is this objective - apart from disabling Bin Laden's terrorist infrastructure? The prime minister describes it as "justice", but has not crystallised this war aim as he did over Kosovo when he called for Milosevic to be brought before an international criminal court. This is understandable, since the US (and particularly the Pentagon) has hitherto opposed the creation of any international criminal tribunal which could conceivably indict an American citizen. But without such an objective, the danger is that the war aim will remain that of getting Bin Laden's "head on a plate". However strong the government believes its evidence to be, this does not amount to justice. Yesterday Osama bin Laden was tried in abstentia by parliament and convicted. The real task ahead is to provide a forum for trying him fairly.